Method for Treating Elongated Metal Product by Heating and Oxidizing the Surface in a Controlled Environment

ABSTRACT

A method and apparatus are provided for making a metal product with improved surface characteristics. The method provides for treating the metal product through a method including: heating the metal product; exposing the outer surface of the metal product to a controlled environment; and oxidising the outer surface of the metal product. The heating may take the metal product to an elevated temperature, the elevated temperature of the metal product being at least 725° C. The method further includes a controlled environment around the metal product which is controlled with respect to the atmosphere around the metal product, and in particular the nitrogen and/or oxygen and/or hydrogen content thereof. The result is a more consistent surface for elongate metal products, having a more consistent colour to the surface, a more durable and hard wearing surface and/or a surface which is not prone to flaking.

This invention concerns improvements in and relating to products andmethods for making them, particularly with reference to the surfaceproperties thereof. Elongate products, such as wire products, are ofparticular interest.

In existing techniques for the production and treatment of extremelyelongate metal products, such as wire products, there are problems withthe colour and consistency of the surface produced.

Attempts have been made to improve the position through the use ofchemical treatments and the additional of salts and the like. However,such approaches have either been unsuccessful, for instance due to theresultant surface flaking off) or undesirable for other reasons (forinstance because of the cost of the extensive modifications to theconventional process that they require to implement).

The present invention has amongst its possible aims to address suchshortcomings. The present invention has amongst its possible aims toprovide benefits to the products without expensive or extensivealteration to the conventional processes.

According to a first aspect of the invention we provide a method fortreating a metal product, the method comprising:

a) heating the metal product;

b) exposing the outer surface of the metal product to a controlledenvironment; and

c) oxidising the outer surface of the metal product.

The method of treating may include a first process stage. The firstprocess stage may provide the heating for the metal product.

The heating may be provided in one or more furnaces. One or more or allof the furnaces may be induction furnaces. The heating may be providedby the temperature profile within the furnace. The heating may take themetal product from an ambient temperature to an elevated temperature.The elevated temperature of the metal product may be at least 725° C.,possibly at least 750° C., preferably at least 850° C., more preferablyat least 875° C., ideally at least 890° C. The elevated temperature maybe at most 1400° C., preferably at most 1380° C. The elevatedtemperature of the metal product may be in the range 700° C. to 1400°C., preferably the range 725° C. to 950° C., more preferably 820° C. to900° C. or still more preferably 825° C. to 855° C.

The controlled environment may be controlled with respect totemperature, for instance such that a temperature range is maintained.

The controlled environment may be controlled with respect to theatmosphere around the metal product, for instance the nitrogen and/oroxygen and/or hydrogen content may be controlled.

The controlled environment may provide one form of controlledenvironmental conditions provided in it. Preferably the controlledenvironment provides a consistent temperature ranges and/or mixtures ofgases within it.

The controlled environment may be provided within a container providedin the first process stage. The container may be a tube or otherelongate container. The wire may pass through the container, forinstance from a container entrance to a container exit. The containerentrance and/or exit may be profiled to correspond to the metal productsprofile, for instance with a small tolerance, for instance so as tominimise gas leakage from the controlled environment.

Separate controlled environments may be provided for separate strands ofthe metal product or a common controlled environment may be provided formultiple strands of the metal product.

The controlled environment may be provided with a gas inlet and/or gasoutlet. The controlled environment may be provided with a gas inletproximal to the inlet for the metal product to the controlledenvironment. The gas inlet and the inlet for the metal product may beseparate from one another. The controlled environment may be providedwith a gas outlet proximal to the outlet for the metal product from thecontrolled environment. Preferably a common outlet is provided.

The gas inlet may feed a gas mixture to the controlled environment froma gas feed system. The gas feed system may be provided with a nitrogensupply and/or oxygen supply and/or hydrogen supply. Preferably one ormore or all of the gases are provided in single element form. Preferablyseparate supplies of nitrogen and oxygen are provided, but a commonsupply, for instance an air supply could be used. The supplies may beprovided by pressurised storage tanks.

The nitrogen supply may be provided at a pressure of at least 10 BAR inthe store, for instance gas cylinders.

The oxygen supply may be provided at a pressure of at least 10 BAR inthe store, for instance gas cylinders.

The hydrogen supply may be provided at a pressure of at least 10 BAR inthe store, for instance gas cylinders.

The gas supplies may connected to a mixer. The mixer may control theproportions of the oxygen and/or nitrogen and/or hydrogen in the gasmixture.

The H₂ to O₂ ratio in the total gas mixture may be between 1.5:1 and4:1, preferably between 1.6:1 and 3.7:1, more preferably between 1.65:1and 3.5:1. The H₂ to O₂ ratio in the total gas mixture may be at least1.5:1, preferably at least 1.6:1, more preferably at least 1.65:1. TheH₂ to O₂ ratio in the total gas mixture may be at most 4:1, preferablyat most 3.7:1, more preferably at most 3.5:1.

According to one preferred form, for instance producing a black surfacefinish, the H₂ to O₂ ratio in the total gas mixture may be between 1.5:1and 2:1, preferably between 1.6:1 and 1.85:1, more preferably between1.65:1 and 1.8:1. The H₂ to O₂ ratio in the total gas mixture may be atleast 1.5:1, preferably at least 1.6:1, more preferably at least 1.65:1.The H₂ to O₂ ratio in the total gas mixture may be at most 2:1,preferably at most 1.85:1, more preferably at most 1.8:1.

According to another preferred form, for instance producing a bluesurface finish, the H₂ to O₂ ratio in the total gas mixture may bebetween 3.2:1 and 3.7:1, preferably between 3.3:1 and 3.6:1, morepreferably between 3.35:1 and 3.5:1. The H₂ to O₂ ratio in the total gasmixture may be at least 3.2:1, preferably at least 3.3:1, morepreferably at least 3.35:1. The H₂ to O₂ ratio in the total gas mixturemay be at most 3.7:1, preferably at most 3.6:1, more preferably at most3.5:1.

The oxygen proportion may be between 0.7% and 2.3% of the total gasmixture, preferably between 0.8% and 2.2%, more preferably between 0.9%and 2.1%.

The hydrogen proportion may be between 3% and 3.9% of the total gasmixture, preferably between 3.2% and 3.7%, more preferably between 3.3%and 3.6%.

According to one preferred form, for instance producing a black surfacefinish, the oxygen proportion may be between 1.7% and 2.3% of the totalgas mixture, preferably between 1.8% and 2.2%, more preferably between1.9% and 2.1%. The hydrogen proportion may be between 3% and 3.9% of thetotal gas mixture, preferably between 3.2% and 3.7%, more preferablybetween 3.3% and 3.6%.

According to another preferred form, for instance producing a bluesurface finish, the oxygen proportion may be between 0.7% and 1.3% ofthe total gas mixture, preferably between 0.8% and 1.2%, more preferablybetween 0.9% and 1.1%. The hydrogen proportion may be between 3% and3.9% of the total gas mixture, preferably between 3.2% and 3.7%, morepreferably between 3.3% and 3.6%.

The nitrogen proportion may represent the balance after the oxygenproportion and hydrogen proportion.

The pressure in the gas mixer may be under 10 BAR, for instance under 5BAR.

The gas mixture may be fed from the gas mixer to a reactor. The gasmixture may be fed to the reactor directly or more preferably via abuffer tank. A catalytic reactor may be provided. The reactor may causethe oxygen and hydrogen to at least partially react. The reactor maycause the production of water. The reaction may consume all of theoxygen.

After the reaction and/or after exiting the reactor, the gas mixture mayhave a temperature of between 100° C. and 280° C., preferably between150° C. and 230° C., more preferably between 165° C. and 210° C. Thetemperature and/or other conditions are preferably such that no liquidwater is present.

The gas mixture may pass through a heat exchanger before reaching thecontrolled environment. The heat exchanger may be used to control thetemperature of the gas mixture, for instance to maintain the gas mixturebelow a temperature limit.

The gas mixture may have a pressure of less than 1 BAR when entering thecontrolled environment, for instance between 0.3 and 0.6 BAR.

The method of treating may provide that the surface of the metal productoxidises in the first process stage, for instance in a first reaction,preferably within the controlled environment. The oxidation may be tothe form Fe₃O₄ and/or according to the reaction form3Fe+4H₂O->Fe₃O₄+4H₂. This oxidation may be completed in the controlledenvironment.

The method of treating may provide that the surface of the metal productis reduced in the first process stage, for instance in a secondreaction, preferably with the second reaction occurring after the firstreaction, preferably within the controlled environment. The reductionmay be to the form FeO and/or according to the reaction of formFe₃O₄+H₂->3FeO+H₂O. This reduction may be completed in the controlledenvironment.

The metal product may be pre-formed before the use of the method oftreating. The method of treating may alter one or more surfaceproperties and/or characteristics of the pre-formed metal product. Themethod of treating may alter one or more internal properties and/orcharacteristics of the pre-formed metal product. The method of treatingmay alter one or more properties and/or characteristics for the whole ofthe pre-formed metal product.

The metal product may be fed to the method of treating from a storagelocation. The storage location may be one or more coils.

Preferably multiple strands of the metal product are fed through themethod of treating in parallel with one another.

The method of treating may include a quenching stage. The quenchingstage may be provided after the first process stage. The quenching stagemay be provided immediately after the first process stage. The quenchingstage may harden the metal product. The quenching stage may increase orprovide Martensite content for the metal product. The quenching stagemay be provided with oil. The oil may be circulated and/or may becooled. The metal product may be immersed in the oil as it passesthrough the quenching stage.

The temperature of the metal product on entering the quenching stage maybe at an exit elevated temperature. The exit elevated temperature of themetal product may be at least 725° C., possibly at least 750° C.,preferably at least 850° C., more preferably at least 875° C., ideallyat least 890° C. The exit elevated temperature may be at most 1400° C.,preferably at most 1380° C. The exit elevated temperature of the metalproduct may be in the range 700° C. to 1400° C., preferably the range725° C. to 950° C., more preferably 820° C. to 900° C. or still morepreferably 825° C. to 855° C. The temperature of the metal product onleaving the quenching stage may be less than 50° C., more preferablyless than 35° C.

The method of treating may include a further immersion stage. Thefurther immersion stage may be provided after the first process stageand/or after the quenching stage. The further immersion stage may beprovided immediately after the quenching stage. The further immersionstage may treat the metal product, for instance to temper the metalproduct. The further immersion may increase the ductility and/or removeinternal stresses in the metal product. The further immersion stage maybe provided with molten lead. The molten lead may be circulated and/ormay be heated. The metal product may be immersed in the molten lead asit passes through the further immersion stage.

The temperature of the metal product on entering the further immersionstage may be less than 50° C., more preferably less than 35° C. Thetemperature of the metal product on leaving the further immersion stagemay be in the range 370° C. to 650° C., more preferably in the range390° C. to 610° C.

The method of treating may include a further quenching stage. Thefurther quenching stage may be provided after the first process stageand/or after the quenching stage and/or after the further immersionstage. The further quenching stage may be provided immediately after thefurther immersion stage. The further quenching stage may harden themetal product. The further quenching stage may be provided with waterand/or oil, for instance as oil dissolved in water. The oil may becirculated and/or may be cooled. The metal product may be immersed inthe oil as it passes through the further quenching stage.

The temperature of the metal product on entering the further quenchingstage may be in the range 350° C. to 650° C. or more preferably 390° C.to 610° C. The temperature of the metal product on leaving the furtherquenching stage may be less than 80° C., more preferably less than 65°C.

The method of treating may provide a metal product cleaning stage, forinstance after the further quenching stage and/or before a productstorage stage. The metal product cleaning stage may remove material fromthe metal product. The metal product cleaning stage may be provide anair wipe.

The method of treating may provide a rust preventer application stage,for instance after the further quenching and/or metal product cleaningstage and/or before a product storage stage. The rust preventer may beapplied by spraying and/or wetting and/or immersion in the rustpreventer.

The method of treating may fed the metal product, after one or moretreatments, to a metal product storage stage, such as one or more coils.

The method of treating may provide that the characteristics and/orproperties of the surface do not alter after the quenching stage,particularly with respect to the oxidisation of the surface. The methodof treating may provide that no or substantially no, for instance lessthan 1%, of the oxidation of the surface of the metal product occursafter the further quenching stage, preferably after the tempering stageand most preferably after the quenching stage.

The first aspect of the invention may include any of the features,options and possibilities set out elsewhere in this document, includingin the other aspects of the invention.

According to a second aspect of the invention we provide apparatus fortreating a metal product, the apparatus including

-   -   a) a heating stage for heating the metal product;    -   b) a controlled environment, to which the outer surface of the        metal product is exposed;    -   c) an oxidising stage for oxidising the outer surface of the        metal product.

The second aspect of the invention may include any of the features,options and possibilities set out elsewhere in this document, includingin the other aspects of the invention.

According to a third aspect of the invention we provide a method fortreating a metal product, the method comprising:

-   -   a) heating the metal product;    -   b) exposing the outer surface of the metal product to a        controlled environment;    -   c) oxidising the outer surface of the metal product;    -   d) quenching the metal product; and    -   e) tempering the metal product.

The third aspect of the invention may include any of the features,options and possibilities set out elsewhere in this document, includingin the other aspects of the invention.

According to a fourth aspect of the invention we provide apparatus fortreating a metal product, the apparatus including

-   -   a) a heating stage for heating the metal product;    -   b) a controlled environment, to which the outer surface of the        metal product is exposed;    -   c) an oxidising stage for oxidising the outer surface of the        metal product;    -   d) a quenching stage for quenching the metal product; and    -   e) an tempering stage for tempering the metal product.

The fourth aspect of the invention may include any of the features,options and possibilities set out elsewhere in this document, includingin the other aspects of the invention.

According to a fifth aspect of the invention we provide a metal product,the metal product being a heated, quenched and tempered metal product.

The metal product may be formed of plain carbon steel and/or steelalloy.

The metal product may be a wire. The metal product may be a rolled wire.The metal product may be a shaped wire. The metal product may be a flatwire. The metal product may have a width of between 0.2 mm and 50 mm,more preferably between 1.5 mm and 22 mm. The metal product may have athickness of between 0.2 mm and 10 mm, more preferably between 0.4 mmand 8 mm.

The metal product may have round edges or square edges. The metalproduct may have a Hardness Rockwell C Scale RC of 20 to 60, preferably35 to 60, more preferably 40 to 55, for instance with a 5 point range.The metal product may have a tensile strength of 1000 to 2000 N/mm²,more preferably 1250 to 1750 N/mm².

The surface of the metal product may not form any flakes, for instancewhen scratched with a knife.

The surface of the metal product may be free from scales, for instancewhen visually inspected.

The surface of the metal product may be consistent in colour, forinstance greater than 90% of the surface being of the same colour,preferably greater than 98%, more preferably greater than 99% andideally greater than 99.9%. The colour may be black. The colour may bedark blue. The colour may be brown or straw.

The metal product, particularly the surface of the metal product mayhave a shelf life of at least 5 days, preferably at least 8 days, morepreferably at least 12 days and ideally at least 20 days, before visiblerust forms on the surface, for instance when stored at 15° C. and 80%humidity.

The fifth aspect of the invention may include any of the features,options and possibilities set out elsewhere in this document, includingin the other aspects of the invention.

Various embodiments of the invention will now be described, by way ofexample only, and with reference to the accompanying figures in which:

FIG. 1 is a schematic illustration of the process stages in aconventional wire treatment process;

FIG. 2 is a schematic illustration of the process stages of a wiretreatment process according to an embodiment of the invention;

FIG. 3 is a schematic detailed illustration of the controlledenvironment of an embodiment of the invention;

FIG. 4 is a schematic illustration of a gas feed system; and

FIG. 5 is a schematic illustration of an alternative gas feed system.

Various techniques are known for the production and treatment ofextremely elongate metal products, such as wire products. A consistentproperty of these production and treatment techniques is that thesurface which forms on the metal product is inconsistent. This canmanifest itself in terms of the colour of the surface in particular. Thesurface is frequently uneven in colour and with a sheen similar to thatseen when oil spreads on water. The inconsistency of the surface canalso manifest itself in terms of the surface being a heavy scale whichhas a tendency to flake off with use and/or time.

The uneven properties of the surface are undesirable in a number ofrespects. The uneven colour impairs the visibility of any markingsapplied to the metal product, such as the distance markings on a tapemeasure. The uneven properties can also shorten the shelf life of theproduct before visible weathering or even rusting has occurred. This isan issue with respect to the storage of such metal products betweenproduction and subsequent use in a further stage.

The present invention has amongst its possible aims to provide a moreconsistent surface for elongate metal products. The present inventionhas amongst its possible aims to provide a more consistent colour to thesurface of elongate metal products. The present invention has amongstits aims to provide a more durable and hard wearing surface and/or asurface which is not prone to flaking. The present invention has amongstits possible aims to provide a surface which is more resistant tooxidation and/or rusting and/or weathering.

In a conventional wire treatment process, FIG. 1, a series of parallelwires 1 are feed through the same treatment stages to give maximum wirethroughput for the process stages provided.

The pre-produced wire 1 is received coiled. The coiled wire 1 is fedfrom the coil 3 into the first process stage 5 and carries on throughall the stages and then on to a coil 27 after the last process stage.Periodically new coils 3 are spliced to or otherwise fed to form thewire strand going to the first process stage 5. Periodically full coilsare cut from the wire strand after the last process stage and new coilsof finished product started.

The first process stage 5 is an induction heated furnace 7. This is usedto raise the temperature of the wire strand to the desired level. Thetemperature profile within the furnace may be used to raise thetemperature to the ultimate level needed. The temperature within thefirst process stage 5 prevents oxidation of the wire 1.

Upon leaving the first process stage 5, the hot wire 1 is quicklyquenched in a quenching stage 9. This is done by immersing the wire 1 ina bath 11 of cooled oil 13. The oil 13 prevents oxidation of the wire 1and the exposure of the wire 1 between leaving the furnace 7 andentering the oil 13 of the oil bath 11 is short enough for there to beno material oxidation of the wire 1.

The combination of heating and quenching is used to give the desiredinternal structure for the wire 1, namely a martensite.

From the quenching stage 9, the wire 1 proceeds into a further immersionstage 15 in the form of a lead bath 17. The molten lead 19 in this bath17 is used to heat the wire 1 and cause tempering. This prevents thebrittleness which would otherwise be present. The heat applied to thewire 1 in the bath 17 together with moisture cause the oxidation of thesurface of the wire 1 before it reaches the next stage.

From the further immersion stage 15, the wire 1 proceeds into a finalquench stage 21. The final quench stage 21 uses water and a soluble oilin combination to reduce the wire temperature quickly.

The cooled wire 1 then passes to an air wipe cleaning stage 23, a rustpreventer application stage 25 and then onto the final coil 27.

Oxidation for the wire occurs primarily after the further immersionstage 15 and before the final quench stage 21.

In the wire production process of the present invention, the minimalchanges possible are made to the overall process, so as to minimise thecapital and operating costs for the revised process. As a result, manyof the stages are the same as in the conventional method and thesequence in which they are provided is the same. However, theenvironment provided and materials added to certain of the stages isgreatly changed and with very significant results.

Once again, the pre-produced wire 1 is received coiled and is fed to thefirst process stage 5. The first process stage 5 is again an inductionheated furnace 7.

In the furnace 7 the wire is heated according to the temperature profileprovided.

Unlike in the previous approach, as shown in FIG. 3, the wire 1 enters atube 53 whose axis 55 is aligned with the direction of movement arrow Aof the wire 1. Only one tube 53 is shown in FIG. 2 for clarity reasons.The tube 53 is provided with an inlet 57 profiled to receive the wire 1and minimise gas mixture 59 loss from inside the tube 53. A similarstructure is provided for the outlet 61 at the end of the first processstage. The wire 1 is inside the tube 53 throughout its time in thefurnace 7.

On entry into the furnace 7, the temperature of the wire 1 is quicklyraised from an ambient temperature, for instance 20° C., to thecontrolled treatment temperature of between 725° C. and 1450° C.

As the wire 1 is passing along the axis of a tube, as shown in FIG. 3,the wire 1 is surrounded by the gas mixture 59 fed to the tube through agas inlet 65. The gas inlet 65 is connected to the gas feed 67 systemdescribed in more detail below. The role of the gas mixture 59 is alsodescribed in detail below.

The tube 53 and wire 1 pass through the furnace and are maintained atthe desired temperature there and prior to reaching the bath 11. As anoption, it would be possible to provide the controlled conditions withina tube 53 provided after the furnace 7 and prior to reaching the bath11, or combinations thereto.

The controlled conditions within the tube 53 mean that oxidation occurshere. The temperature and the controlled conditions control theoxidation. This is a fundamental difference compared with theconventional approach where oxidation occurs in the much later stagesand the furnace is just used for heating.

Upon leaving the first process stage, and the tube 53, the hot wire 1 isquickly quenched. This is done by immersing the wire 1 in a bath 11 ofcooled oil 13. The oil 13 and temperature reduction prevent furtheroxidation. During the short exposure between leaving the furnace 7 andentering the oil 13 some further oxidation may occur due to theconditions the wire 1 has already been exposed to and/or the reactionsalready underway.

The combination of heating and quenching is used to give the desiredinternal structure for the wire 1, namely a martensite, as in theconventional process. However, the surface has been materially alteredrelative to the surface at this stage in the conventional process.

From the quenching stage 9, the wire proceeds into a further immersionstage 15 in a lead bath. The molten lead 19 in this bath 17 is used toheat the wire 1 and cause tempering. This prevents the brittleness whichwould otherwise be present.

From the further immersion stage 15, the wire proceeds into a finalquench stage 21. The final quench stage 21 uses water and a soluble oilin combination to reduce the wire temperature quickly.

The cooled wire 1 then passes to an air wipe cleaning stage 23, a rustpreventer application stage 25 and then onto one of the final coils 27.

Unlike in the conventional process, the oxidation for the wire occursprimarily in the earlier stages of the process and no, or substantiallyno, oxidation occurs after the final quench stage 21.

As mentioned above, the gas inlet 65 receives a gas mixture 59 from agas feed system 67, as shown in FIG. 4. The gas feed system 67 isprovided with a nitrogen storage tank 69, a hydrogen storage tank 71 andan oxygen storage tank 73. The three storage tanks 69, 71, 73 areconnected to a gas mixer 75. The nitrogen fed to the gas mixer 75 fromthe compressed gas cylinders, reduces in pressure from around 300 BAR(from a full cylinder) to 9.5 BAR in the gas mixer 75. The hydrogen fedto the gas mixer 75 from the compress gas cylinders, reduces in pressurefrom 172 BAR to 9.0 BAR.

The gases are mixed in the gas mixer 75 to give the gas mixture 59 inthe form of 2.7% H₂+1.2% O₂+96.1% N₂. The gas flow is reduced to 10-20m³/h and introduced, via a buffer tank 77, to a catalytic reactor 79.The gas mixer 75 is connected to the buffer tank 77 to ensure a steadysupply of the gas mixture 59 to the controlled environment in the tube53.

In the catalytic reactor 79, the oxygen is reacted with the hydrogen. Asa result of this reaction, H₂0+H₂+N₂ forms the gas stream and thetemperature average of 204° C. is produced.

If higher temperatures for the gas stream are desired, then a heatexchanger (not shown) can be used. The gas valves and gas regulators areunable to handle the gas stream as hot as 230° C. and so the heat wouldbe introduced after it had passed through those pieces of equipment, butbefore entering the tubes 53.

When the gas mixture 59 is introduced to the tubes 53 which define thecontrolled environment in the furnace, the gas mixture 59 is at 80° C.to 230° C. and the gas mixture 59 has a dew point of around 20° C. Thecombination of the gas temperature and the dew point means that the H₂0is maintained as steam within the tubes 53.

All the oxygen from the water is used in the first oxidation reaction.This reaction starts once the necessary temperature threshold is reachedand given the presence of the gas mixture. This part of the process alsoprovides some initial hardening. The reaction can take place intemperature range of 575° C. to 1377° C. and is of the form3Fe+4H₂O->Fe₃O₄+4H₂. A temperature of 900° C. may be used.

The H₂ in the gas mixture is used for the second reaction. This secondreaction starts once the first reaction occurs and increases as theamount of the surface for which the first reaction has occurredincreases. This part of the process also provides some further hardeningand there is reduction of Fe₃O₄ to FeO.

A gas removal hood may be provided over the bath 11 of cooled oil 13 tocapture the gaseous reaction products and unreacted part of the gasmixture. This can assist in drawing off the gas mixture and stopping anyfurther reactions.

In an alternative form, the hydrogen storage tank can be used with anair storage tank to provide the gas feed. The gases when supplied inthis form are less easily varied to provide some of the process controlcharacteristics, however, and so the three separate gas stores arepreferred. For instance, the dew point is materially different in airwhen compared with that for oxygen. In such a case, the compressed air(which is 20% O₂+80% N₂) is fed to the gas mixer from the compress gascylinders and the pressure reduces from 200 BAR to 9.0 BAR.

In the alternative gas feed system shown in FIG. 5, the operation isprovided for in a similar manner to FIG. 4, but with the oxygen orcompressed air being fed direct from the storage tank 73 to the reactor79.

In terms of the process conditions applying during these stages:

-   -   a) The wire 1 is uncoiled at 20° C. or the ambient temperature        in the process plant.    -   b) The induction furnace is used in the first furnace stage to        heat the wire from to 20° C. up to 550° C.    -   c) In the tubes, the wire is isolated from the ambient        atmosphere and subjected to the controlled atmosphere. The tubes        are also insulated to maintain the wire temperature at the        desired level of 550° C.    -   d) The controlled gas mixture is introduced to the tubes with        the gas at a temperature of between 20° C. to 204° C.    -   e) In the second furnace stage, the wire and controlled        environment is heated to 575° C. This provides the first        oxidation phase and generates a consistent oxidation layer on        the surface which is blue in colour. The surface is oxidised to        Fe₃O₄.    -   f) In the third furnace stage, additional heat is used to raise        the temperature to 900° C. This provides a reduction phase and        generates a consistent oxidation layer on the surface which is        black in colour. The surface is reduced from Fe₃O₄ to FeO by the        H₂ reduction.    -   g) In the quench stage the wire temperature drops from above        725° C. (typically 840 to 900° C.) to 30-35° C. and the quench        oil causes the formation of the desired uniform martensite        structure.    -   h) The lead bath stage is then used to temper the wire by        heating the wire again to 400 to 600° C.    -   i) The second quench stage uses a water and soluble oil quench        to take the wire temperature back down to 30 to 60° C.    -   j) The air wipe stage is used to clean the wire and to cool it        further to 20 to 25° C., or ambient conditions.    -   k) The rust preventative is applied at 20° C., or ambient        conditions.    -   l) The finished wire in the coiling stage is at 20° C., or        ambient conditions.

In the above method, the temperature and gases in the controlledenvironment are controlled so as to provide a uniform black surface forthe wire product. Other surface colours are possible through alternativeconditions.

The black colour is caused by the formation of FeO as the surfacematerial. This is based upon the proportion of H₂ converted to H₂O beingcorrect for the oxidation to black FeO as the oxide form.

If the level of H₂ is lower, then the second phase will not occur andthe oxide will be in the form after the first oxidising phase, namelyFe₃O₄.

If the level of H₂O supplied to the first oxidation phase is not highenough then a brown or straw colour oxide will form. This is a result ofthe reaction 2Fe+3H₂O->Fe₂O₃+3H₂ (instead of 3Fe+4H₂O->Fe₃O₄+4H₂).

Hence, control of the hydrogen level and control of the oxygen level canbe used to control the colour of the product formed. Other variationsare anticipated to provide further colours or shades of colour.

In addition to the surface colour control, the position at whichoxidation occurs in the overall process is changed. This means thatwhilst in the conventional process the wire can be exposed to air in twoplaces late in the process sequence, in the new process all the wiresurface is fully oxidised prior to reaching these locations and it isnot possible for another oxidation to occur as all Fe is protected withby the FeO surface layer which has already fully formed.

Detection of any problems with the black oxide layer is apparent fromthe blue or straw colour forming should there be any surface damage inthe lead tank, for instance, causing later air oxidation.

1. A method for treating a metal product, the method comprising: heatingthe metal product; exposing the outer surface of the metal product to acontrolled environment; and oxidising the outer surface of the metalproduct.
 2. The method according to claim 1, the method furthercomprising heating the metal product to an elevated temperature, theelevated temperature of the metal product being at least 725° C.
 3. Themethod according to claim 1, the method further comprising thecontrolled environment being controlled with respect to the atmospherearound the metal product.
 4. The method according to claim 3, the methodfurther comprising the nitrogen and/or oxygen and/or hydrogen contentbeing controlled.
 5. The method according to claim 1, the method furthercomprising the controlled environment being provided within a containerprovided in a first process stage, the container being a tube or otherelongate container, and the metal product passing through the container.6. The method according to claim 1, in which a gas mixture is fed to thecontrolled environment and in which the H₂ to O₂ ratio in the total gasmixture is between 1.5:1 and 4:1.
 7. The method according to claim 1, inwhich a gas mixture is fed to the controlled environment and in whichthe H₂ to O₂ ratio in the total gas mixture is between 1.65:1 and 3.5:1.8. The method according to claim 1, in which a gas mixture is fed to thecontrolled environment and in which the H₂ to O₂ ratio in the total gasmixture is between 1.5:1 and 2:1
 9. The method according to claim 1, inwhich a gas mixture is fed to the controlled environment and in whichthe H₂ to O₂ ratio in the total gas mixture is between 3.2:1 and 3.7:1.10. The method according to claim 1, the method further comprising thatthe surface of the metal product oxidises in a first process stage. 11.The method according to claim 10, in which the oxidation issubstantially completed in the controlled environment.
 12. The methodaccording to claim 1, the method further comprising that the surface ofthe metal product is reduced in a first process stage.
 13. The methodaccording to claim 12, in which the reduction is substantially completedin the controlled environment.
 14. The method according to claim 1, inwhich the method further comprises a quenching stage, the quenchingstage being provided after the first process stage, the quench stagehardening the metal product.
 15. The method according to claim 1, inwhich the method further comprises a further immersion stage, thefurther immersion stage being provided after the first process stageand/or after the quenching stage, the further immersion stage temperingthe metal product.
 16. The method according to claim 1, in which themethod further comprises a further quenching stage, the furtherquenching stage being provided after the first process stage and/orafter the quenching stage and/or after the further immersion stage, thefurther quenching stage hardening the metal product.
 17. The methodaccording to claim 1, in which the method of treating provides that thecharacteristics and/or properties of the surface do not alter after thequenching stage, particularly with respect to the oxidisation of thesurface.
 18. The method according to claim 17 in which the methodprovides that less than 1% of the oxidation of the surface of the metalproduct occurs after the further quenching stage, preferably after thetempering stage and most preferably after the quenching stage. 19.Apparatus for treating a metal product, the apparatus comprising: aheating stage for heating the metal product; a controlled environment,to which the outer surface of the metal product is exposed; an oxidisingstage for oxidising the outer surface of the metal product.
 20. A methodfor treating a metal product, the method comprising: heating the metalproduct; exposing the outer surface of the metal product to a controlledenvironment; oxidising the outer surface of the metal product; quenchingthe metal product; and tempering the metal product.
 21. Apparatus fortreating a metal product, the apparatus comprising: a heating stage forheating the metal product; a controlled environment, to which the outersurface of the metal product is exposed; an oxidising stage foroxidising the outer surface of the metal product; a quenching stage forquenching the metal product; and an tempering stage for tempering themetal product.